1. Field of the Invention
This invention concerns a variable-venturi carburetor of the type having a throttle valve which is provided downstream in a bore, a venturi portion, a suction chamber which has a rod guide and a suction piston which is disposed perpendicularly to the venturi portion. The invention relates more particularly to such a carburetor which has a rod slidably supported by the rod guide, a negative pressure chamber which is formed in the suction chamber by the suction piston, and a suction spring which is interposed between the suction chamber and the suction piston.
2. Description of the Prior Art
As is well known, as to carburetors which are attached to an engine in a vehicle such as an automobile, there are a fixed-venturi carburetor and a variable-venturi carburetor. The latter has various advantageous merits that the device height is low and that fuel metering can be made by the use of only a metering needle and a metering jet, and accordingly, a superior transient characteristic can be obtained due to less number of junctions as compared with the fixed-venturi carburetor. Thus, the variable-venturi carburetors have been widely used in practical vehicles, and further improvements and developments are being made at present and expected from now on.
In recent times, however, public pollution problems, especially problems about exhaust, gas from vehicles, have been taken up as serious problems. In the field of automobiles, with stricter regulation against public pollution, wider use of the so-called three-way catalytic converter (catalyst) is expected as a matter of course, which can treat three components of exhaust, namely HC, CO and NOx contained in the exhaust gas, changing these components into non-polluting materials at the same time in the exhaust system.
In order to permit the three-way catalytic converter to effect its full function, it is necessary to maintain the air-fuel ratio of exhaust gas, which is to encounter with the catalyst, as near as possible to the theoretical ratio of A/F=14.7. For this purpose, various devices, such as a feedback control mechanism utilizing O.sub.2 sensor, have been proposed and developed.
Such prior art air-fuel ratio control techniques can properly control the air-fuel ratio as designed when the engine is running under the normal air-fuel ratio characteristic. Such prior art mechanisms, however, cannot assuredly offer proper control during transient stage because of time-lag wherein the mechanism cannot follow variation in air-fuel ratio, and accordingly air-fuel ratio characteristic is ruined.
In fact, as is well known, there are many acceleration and deceleration modes in actual automobile running on roads and also in the exhaust gas measurement modes which simulate the actual running. Accordingly, the above-described failure to effect the maintaining of the air-fuel ratio, within acceptable limits, caused during the transient stage is inevitable.
Accordingly, without some improved technique against the transient stages, sufficient countermeasure against exhaust gas will not be achieved.
For reasons as described above, the variable-venturi carburetor, which is superior in transient characteristic to the fixed-venturi carburetor, is now being identified as an effective apparatus having potential for the above-mentioned exhaust gas countermeasure.
When a throttle valve in conventional variable-venturi carburetors, which are usually provided downstream in a bore formed by a barrel, makes opening-and-closing operation by rotational movement thereof about its throttle shaft, a corresponding amount of air will, in response to such operation of the valve, be taken in from an air horn, which is provided upstream, and flow down through a venturi portion. With such air flow, a suction piston, which is conventionally slidably supported on a rod fitted in a rod guide extending into a suction chamber provided on one side of the barrel and is adapted to form a negative pressure chamber within the suction chamber, will be moved forward and backward. A suction spring interposed between the suction piston and the suction chamber, and the atmospheric pressure in an atmospheric air chamber which is in communication through a communication passage with the air horn is provided. Such movement of the suction piston will, in its turn, change the sectional area of the venturi portion. Concurrently with the movement of the piston, a metering needle, which is secured to the head by forcing the base portion of the needle into the head, will move forward and backward passing through a main nozzle. A metering jet is provided in the barrel while centering. In this manner, the metering needle will, in cooperation with the metering jet, meter fuel which is sucked in from a float chamber through a suction pipe and a fuel passage. This permits the fuel to spout out from the main nozzle together with bleed air drawn in through an air bleed passage which is communicated with the air horn. Such spouted fuel will then be mixed with the air flowing down through the venturi portion to form mixture of atomized fuel particles and air which will then be sent through a mixing chamber and a throttle bore into a fuel chamber. A passage for draining residual fuel is provided in the negative pressure chamber.
Differing from a fixed-venturi carburetors, variable-venturi carburetors, have a throttle bore having a sectional area two or three times as large as that of the low-speed throttle bore of fixed-venturi carburetors.
Accordingly, the sectional area of the venturi portion during operation becomes smaller to permit as smaller amount of mixed gas of fuel and air to be taken in, the mixed gas passing through the venturi portion at a speed of as high as several tens of m/sec. The speed of the gas, however, in the throttle bore will be rapidly decreased to several m/sec which is very low as compared with that in the fixed-venturi carburetors.
As a result, atomization of fuel can be enhanced by the high-speed gas stream to produce good mixing of atomized particles of fuel and air in the venturi portion. On the other hand, in the throttle bore or in the throttle bore and in the vicinity of the mixing chamber, a considerably large amount of fuel will deposit on the wall of the main bore and/or on the lower surface of the suction piston in a wet state and will drip down as irregular drips, thus causing variation in the normal air-fuel ratio characteristic and injuring operational performance. Further, in the transient stage, the amount of the deposited fuel itself will vary, thus injuring the normal, intrinsic transient characteristic of the variable-venturi carburetor, resulting in a lean state at the time of acceleration and in a rich state at the time of deceleration. Such problems as the above have been a bottle neck against maintenance of a correct air-fuel ratio which is required, as described above, for effective action of the three-way catalyst.